Johne's disease, or paratuberculosis, is a fatal chronic granulomatous enteritis of animals caused by Map. The disease is characterised by severe emaciation, loss of body condition and in some species diarrhea. The disease is mainly spread through the ingestion of faeces from an infected animal. Infected animals can also pass on the infection in colostrum or milk and across the placenta to unborn animals. It is generally believed that young animals are more susceptible to infection than adults. Following infection there is a long incubation period of 2 to 4 years during which time the animal may show no signs of clinical disease and may shed Map intermittently. Such animals are often described as “subclinically infected” and act as “carriers” of the disease. The disease is usually introduced to a farm through the purchase of subclinically infected stock.
Johne's disease occurs worldwide and causes considerable economic losses through decreased productivity, increased wastage of adult animals as well as the cost of control, monitoring and diagnosis. There is also some controversy as to whether Map is involved in the development of Crohn's disease. Contaminated milk would constitute a source of infection and there is a drive towards elimination of Map from the food chain.
The diagnosis of Johne's disease is problematic and there is no single diagnostic test that can detect all stages of the disease. Subclinically infected animals are particularly difficult to diagnose and results of currently available tests may be negative. The most commonly used diagnostic test is the serum Enzyme Linked Immunosorbent Assay (ELISA). This detects circulating antibodies to Map in infected animals. It will detect animals in the later stages of the disease, where clinical symptoms are often present, which are generally those shedding large numbers of Map. It will not, however, detect animals in the early stages of infection, typified as being subclinical where the antibody levels are below the sensitivity threshold of the test. The other most commonly used tests are faecal smears and bacteriological culture. Faecal smears detect the presence of acid-fast bacteria in the faeces and probably only detect a third of the infected animals in a herd/flock. Bacteriological culture is more sensitive and specific but takes 6 weeks or more which is less than desirable. Moreover, low shedders can sometimes be difficult to detect and subclinically infected animals are often missed because they shed intermittently.
PCR-based tests are not used routinely as yet but are available for detecting Map in milk, blood or faeces. Also there are particular problems with the application of PCR tests to milk, blood and faecal samples. There is therefore a requirement for a test that can detect early infection and subclinically infected animals.
It is amongst the objects of the present invention to obviate and/or mitigate at least one of the aforementioned disadvantages.